1. Field of the Invention
This invention relates to a method of manufacturing granulated metal from fused ferronickel, partiailarly, to method of manufacturing granulated metal suitable for producing alloy steel.
2. Description of the Prior Art
For producing granulated metal from fused metal various methods have hitherto been proposed or put in practice; for example, one in which high pressure water is jetted against a fused metal stream to crash the fused metal (Japanese Patent publication No. 8675/1978), one which a fused metal stream is allowed to fall into a water pond while blowing air jets against the fused metal stream immediately below the water surface to obtain a shot thereby (Japanese Patent lay-open Print No. 66468/1975), and one in which granulation is effected by producing horizontal water streams directed in different directions within a water pond and pouring fused metal into the water pond (Japanese Patent publication No 34309/1973).
In the manufacture of granular metal from fused ferronickel by one of the well-known water granulation methods mentioned above, it is possible to obtain granules which can be sufficiently put in practice by appropriately selecting the water granulating means in case of fused ferronickel with low carbon content. However, in case that the carbon content in the fused ferronickel is above a certain value, granular metal can no longer be obtained, but there result indefinite shapes, for instance greatly wrinkled thin flat forms just like what is obtained by crumpling paper into a ball and then lightly expanding it, those like a Jew's ear (Auricularia auriculajudae) or those having many projections, even by employing any sort of water granulating means.
Such shapes, as shown in FIG. 3 for instance, are naturally low in bulk specific gravity and are likely to get entangled, thus posing problems in discharging from a hopper by definite quantities, transportation by conveyor means, drying after water granulation and so forth. Further, in case of using such metal, for instance, as cold material in the manufacture of stainless steel, in which case it is thrown into raw stainless steel melt, if the melt contains slags, it floats on the slags because its bulk specific gravity is low, so that its quick melting cannot be expected.
A recent trend in a stainless steel manufacturing process is to replace the conventional electric arc furnace process with an AOD (argon oxygen decarburizaton) process, that is, an Ar-O.sub.2 gas bottom blow process in accordance with the advancement of the electric furnace and peripheral techniques.
The AOD process can overcome such drawbacks inherent in the electric arc furnace process as long decarburization treatment at a high temperature (of about 1,800.degree. C.), accompanying conversion of chromium into slay, reducing step for preventing loss of chromium and further limitation imposed upon the material (i.e., incapability of treating materials of high carbon content), because it permits to keep the CO partial pressure low within the furnace, to treat materials even of high carbon content, to reduce the period required for the manufacture of steel practically to one half and reduce the loss of chromium.
This AOD process, however, has a drawback that the furnace temperature is increased (to 1,800.degree. C. or above) at the time of the decarburization treatment, so that corrosion of lining by fusion is liable to result due to this high temperature. Therefore, it is necessary to add a suitable cold material at the time of the decarburization. As the cold material it is possible to use granular low carbon ferronickel and ferrochromium and iron pieces which have been used as material or composition adjustment agent in the prior-art electric arc furnace process, but they are expensive. Stainless steel scrap might be thought as an alternative, but it is bulky and thus requires some treatment.
Accordingly, there has been a strong demand for high carbon ferronickel, which can serve both as cold material and as composition adjustment agent as well as being inexpensive, small in grain size and convenient to handle, that is, solid granular shots of sufficient flow property.